This invention relates generally to food processing and more particularly to processes for producing various types of food juices which can be stored.
Fresh fruits and vegetables are generally available only during specific growing seasons. Thus, fruits and vegetables are processed while they are available to produce juice which may be stored in order to provide a year round supply.
In processing various types of fruits and vegetables, it is desired to produce juices which retain the characteristics of fresh juice in order to market a product acceptable to consumers. As a result, processors strive to produce juices which retain as much as possible the flavor, aroma, appearance, and mouth feel of fresh juice.
Unfortunately, processes which produce juices suitable for storing involve steps which detract from these desirable characteristics. Juices contain aroma and flavor components which are of low molecular weight and are easily volatilized at temperatures above 40.degree. C. But producing a juice which can be stored generally requires pasteurization to destroy spoilage microorganisms which can cause complete loss of the product or result in undesirable off-flavors and odors. Pasteurization typically employs heating to 62.degree. C. for about 30 minutes, although higher temperatures for shorter periods may be employed. Consequently, these volatile flavor and aroma components are lost, detracting from the flavor and aroma of the juice.
In addition, fruits and vegetables contain enzymes which can cause off-flavors, off-aromas, loss of color or other undersirable characteristics. For example, many fruits and vegetables contain pectin which provides the desirable characteristic body and cloud of many fresh juices such as orange and grapefruit juice. However, many fruits and vegetables also contain pectinesterase, an enzyme which if not inactivated will cause the loss of the desirable body and cloud provided by pectin. This is due to the deesterification of pectin and the subsequent precipitation of methoxyl pectin as calcium pectinate or pectate which results in the clearing of the juice and a noticable layer of precipitate at the bottom. Thus, the juice is heated to inactivate the pectinesterase if the juice is of the type where "cloud" is desired. Disadvantageously, the temperatures required to inactivate pectinesterase are higher than the temperatures required for pasteurization, ranging from approximately 48.degree. to 98.degree. C. Consequently, the loss of flavor and aroma components is further compounded.
Heating juice during processing can also result in the oxidation of compounds in the juice, especially if dissolved air is present. If flavor and aroma compounds are oxidized, the resulting degradation products can produce undesirable off-flavors in the final product.
Processes which involve heating are generally employed in producing "canned" juices suitable for storage at room temperature, chilled juices requiring refrigeration, and frozen concentrated juices. These are typical, for example, of the orange juice industry.
In the orange juice industry, orange juice is produced by passing extracted orange juice through a heating stage to inactivate pectinesterase enzymes and destroy microorganisms. The temperatures typically employed are at least 90.degree. C. in order to inactivate the pectinesterase. In producing chilled juices, which generally require refrigeration at 10.degree. C. or below, the juice is sterilized by rapid heating to about 115.degree. C. for a few seconds followed by a rapid cooling to about 4.degree. C. before filling.
The juice industry's most widely distributed processed citrus product is concentrated frozen orange juice. The conventional method of preparing an orange juice concentrate is by evaporation concentration. This is generally done by thermally accelerated short time evaporation (TASTE) units. In this process, juice passes through preheaters which destroy microorganisms and enzymes. The juice then passes through several stages of evaporators. The actual time spent in evaporation is generally on the order of 6 to 8 minutes. After the last stage, the juice is flash cooled to below 10.degree. C.
In evaporation concentration a significant portion of the various volatile alcohols, esters, and aldehydes which constitute a portion of the flavor and aroma components of juice come off with the first 15 to 20% of the water vaporized. This is referred to as the "essence." Loss of these flavor and aroma components cause significant deterioration in the quality of the juice. However, some of the aqueous essence can be recovered from the first stage of the evaporation process by concentrating the essence in fractionating columns and adding it back to the final concentrate to improve flavor. Still, only a fraction of the compounds are recovered, resulting in a net loss of the overall flavor and aroma of the juice.
Alternate methods of producing juice concentrates without subjecting the flavor and aroma components to heat have also been developed. These methods employ freeze concentration or sublimation concentration.
In freeze concentration, extracted juice is centrifuged to separate a pulp portion and a serum portion. The serum portion is freeze concentrated and the concentrate is added back to the pulp portion. However, in this process, aroma and flavor compounds are entrained in significant proportions in ice crystals separated from the freeze concentrate resulting in a loss of flavor and aroma components and a decrease in the quality of the product.
In sublimation concentration the extracted juice is separated into a pulp and a serum portion as in freeze concentration. Water is removed from the serum as pure vapor using a freeze drying apparatus.
In both freeze concentration and sublimation concentration, undesirable oxidation products can result which impart an off-flavor. However, in one method, oxidative degradation is claimed to be reduced by using an inert atmosphere in a closed system, although unfortunately this system appears to be economically inefficient for large-scale applications due to the added expense of a closed system and the costs of freezing.
Although processes involving freeze concentration and sublimation concentration have claimed retention of at least 65% of the volatile flavor compounds, greater retention of flavor and aroma components is desired.